An electrochemical cell, in its simplest terms, consists of an anode (the oxidizing electrode), a cathode (the reducing electrode) and an electrolyte. In order for the electrochemical cell to function, the electrolyte must be compatible with the mechanisms of oxidation and reduction at the electrodes. As well, it must provide a conductive path for the transport of ionic species between the electrodes.
The electrochemical cell concept is broadly applied in industrial and scientific operations. Electrolytic cells are used in electroplating, water purification, and the production of high purity gases and metals, while electrochemical cells, such as batteries and fuel cells provide a convenient means of energy storage and generation.
Also, due to their very high level of sensitivity, electrochemical cells are used for measurement in a variety of analytical procedures and many laboratory and process control instruments depend on the electrochemical cell as the sensing element for their function.
U.S. Pat. No. 4,960,497 discloses a system wherein an electrolytic cell measures oxygen in the ppb range. In this system, the dissolved oxygen in the electrolyte is removed to allow for an accurate reading of the oxygen concentration in a gas sample. However, in this system, when measuring in the 0-100 ppb range, it was found that in some instances the signal-to-noise ratio was not high enough to provide a consistently accurate reading.
There are other electrochemical systems currently available which measure oxygen in the 0-100 ppb range. These sensors are known as galvanic or battery type systems that use lead or cadmium as the anode. In these systems, as oxygen is measured by the sensor, the lead or cadmium anode is consumed. There are several inherent drawbacks in the use of these consumable anodes. One drawback is that the critical cathodic potential is determined directly by anodic potential, since these systems are galvanic (no applied potential). The anodic potentials of lead and cadmium drift as the electrodes are consumed thus affecting the stability of the critical cathodic potential. Drift in the cathodic potential will result in calibration drift. A second drawback to using lead or cadmium is that as the anodes are consumed they produce byproducts that are soluble in the electrolyte solution. These byproducts are free to migrate to the cathode and contaminate the electrode surface causing further calibration drift.
U.S. Pat. No. 5,256,273 disclosed a stable electrochemical system and a method for measuring an analyte, i.e. oxygen in the 0-100 ppb range. The system functioned as a hydrogen-oxygen alkaline fuel cell configured to generate current which was linear to the rate at which the analyte was either reduced at a cathode, i.e. oxygen or oxidized at the anode, i.e. hydrogen. That system provided consistently accurate readings in the 0-100 ppb range for oxygen. The system exhibited very little calibration drift because the anode was not consumed during measurement and therefore maintained a stable potential. A second reason for the excellent calibration stability can be attributed to the fact that there were no soluble byproducts of the hydrogen anode reaction. However, the presence of a hydrogen source with the system in some instances raised safety considerations.
The present invention embodies a system and a process for measuring oxygen, particularly in the 0-100 ppb range. The system is a polarographic system using an anode which is specifically designed for long-term potential stability, high electrochemical reversibility and chemical inertness. For oxygen analysis, the cathode used is the same as the prior art system described in U.S. Pat. No. 5,256,273; namely, the cathode is a non-depleting carbon Teflon electrode catalytically specific for oxygen reduction. At the anode an electrochemical oxidation reaction occurs but there are no soluble byproducts that contaminate the electrolyte. The reaction at the anode is a simple oxidation state change of nickel in a nickel composite matrix. As current flows through the sensor during oxygen measurement, the ratio of Ni+2/Ni+3 changes in the composite matrix, however, the anodic potential remains very stable. The composite matrix is designed for stability in KOH which is the preferred electrolyte. This anode reaction is highly reversible meaning that the anodic potential will not change as a function of the current produced by oxygen measurement. By operating this system polarographically, the cathodic potential can be adjusted to any level which is deemed optimal for maximum signal-to-noise ratio. The amount of nickel actually oxidized during usage is so minimal that in essence, the anode is nondepleting.
In the preferred embodiment of the invention, a nickel electrode specifically having the composition 50% Ni(OH).sub.2 and 50% NiOOH is used. An anode used in the system of the invention having a surface area of 8 in.sup.2 and a weight of six grams would be expected to perform satisfactorily when measuring in the sub 100 ppb range of a period approaching 30 years. If desired, the nickel anode can be restored in situ back to its original Ni+2/Ni+3 ratio.
In the preferred embodiment, the invention comprises an polarographic electrochemical cell. A gaseous stream containing the oxygen to be measured contacts a cathode catalytically optimized for oxygen. The oxygen is reduced forming hydroxyl ions. The hydroxyl ions react with the metal anode. The metal anode is oxidized. Collectively these reactions generate a current which is proportional to the rate at which oxygen is reduced at the cathode. The current measured corresponds exactly to the changing concentration of oxygen in the gaseous stream.
Although the preferred embodiment uses a nickel electrode with a composition of 50% Ni(OH).sub.2 and 50% NiOOH, other metal anodes believed suitable for purposes of the invention include MnO.sub.2 /MnOOH, Ag.sub.2 O/Ag.sub.2 O.sub.2, and Hg/HgO.
One advantage of this invention is that the system is a clean system, the byproduct of the anodic reaction is simply a change in the oxidation state of the metal composite anode. With a lead anode or a cadmium anode there are discrete chemical reaction byproducts which build up in the sensor cell and act as inhibitors to the electrochemical reactions.
Distinct advantages over the previous ppb oxygen sensor system disclosed in U.S. Pat. No. 4,960,497 include higher oxygen sensitivity, lower background offset, less offset drift, improved linear response, improved speed of response and reduced temperature sensitivity.